Transimpedance amplifiers are commonly used for providing voltage signal proportional to current signal; they are normally implemented by providing a feedback resistor across the input and output nodes of an operational amplifier.
When utilized in optical communication or optical transmission systems, a transimpedance amplifier converts input optical signal into output voltage signal. In these applications, the optical signal, normally transmitted via optical fibers, is received by a PIN or avalanche photodiode coupled to an input node of the amplifier. The photodiode converts the optical signal into current signal and provides it to the amplifier. Consequently, the amplifier provides at its output terminal voltage signal proportional to the optical signal.
A transimpedance amplifier's merit is directly related to the value of the feedback resistor. For example, the amplifier's sensitivity is proportional to the value of the feedback resistor, whereas the amplifier's bandwidth is inversely proportional to the value of the feedback resistor. Additionally, since the output voltage from the amplifier is a direct product of the input current times the value of the feedback resistor, the amplifier's dynamic range (i.e. the input current the amplifier is capable to handle without incurring significant distortion) is inversely proportional to the value of the feedback resistor.
The dependency of the amplifier's merit on the value of the feedback resistor presents unique problems in optical communication or optical transmission systems. In those applications, it is desirable that the transimpedance amplifiers are fabricated prior to their installation, and that the same kinds of transimpedance amplifiers are used at locations where the optical cables connecting the amplifiers and the optical sources (e.g. lasers or light emitting diodes) are short and at locations where the optical cables are long. Longer optical cables result in greater attenuation of optical signals.
Consequently, if a large value feedback resistor is chosen for high optical sensitivity, the amplifier's bandwidth as well as its dynamic range will be compromised. On the other hand, if a small value feedback resistor is used to obtain large bandwidth and dynamic range, low sensitivity and reduced signal-to-noise ratio results; further, if the value of the feedback resistor is too low, the transimpedance amplifier becomes unstable and begins to oscillate.
When using transimpedance amplifiers for analog optical communications, it is also required that the transimpedance amplifiers exhibit high linearity, i.e. the output signal waveform closely resembles the input signal waveform. This is because, in analog applications, any non-linearity will directly affect the final result (e.g. picture images) of the communications.
U.S. Pat. No. 4,218,613 issued to Bletz discloses a transimpedance amplifier for detecting optical signal. The amplifier includes an operational amplifier, a photodiode having one end connected to the input terminal of the operational amplifier, a feedback resistor connected between the input and output terminals of the operation amplifier, and a diode having non-linear current-characteristics connected in parallel to the feedback resistor. This amplifier offers a greater dynamic range than a conventional transimpedance amplifier because the diode becomes more conductive as the input current increases, which reduces the feedback resistance. However, due to the diode's non-linear current characteristic, distortion in the output signal results.
U.S. Pat. No. 4,620,321 issued to Chown refers to a transimpedance amplifier using an operational amplifier and a Schottky diode ("SD") connected in parallel to a feedback resistor as feedback. The Schottky diode is forward biased and becomes more conductive when the input current increases. However, this amplifier also results in significant non-linear distortion due to the Schottky Diode's non-linear, exponential current characteristics. In addition, the bandwidth of the amplifier is limited by the large capacitance associated with a Schottky diode.
U.S. Pat. No. 4,679,251 issued to Chown discloses a transimpedance amplifier for digital application utilizing an operational amplifier and a resistor connected in parallel to a serially connected Schottky diode and PIN diode as feedback. As a result of the serially connected Schottky and a PIN diode, the bandwidth of the amplifier is improved due to the reduced total feedback capacitance. However, this amplifier still suffers from non-linear distortion as the result of the Schottky diode's non-linear characteristics.
It is therefore an object of the present invention to provide, for analog application, a transimpedance amplifier having a large dynamic range;
it is another object of the present invention to provide a transimpedance amplifier having high linearity;
it is a further object of the present invention to provide a transimpedance amplifier having a wide bandwidth; and
it is still a further object of the present invention to provide a transimpedance amplifier with a variable feedback resistance and high linearity.